The first metal-shrouded two-dimensional semiconductor, single-layer Tl₂O, is discussed from first principles. It is thermally and dynamically stable, has a low cleavage energy calling for exfoliation from layered Tl₂O bulk, and has a very small interface mismatch compared to (001) Tl metal. Single-layer Tl₂O exhibits a direct bandgap of 1.56 eV and a very high charge carrier mobility of 4.3 × 10³ cm² V^-1 s^-1. The metal-shrouded 2D semiconductor promises interesting applications in 2D electronics. An intriguing layer-thickness-dependent direct-to-indirect bandgap transition is observed, and contrary to early literature, the bulk is also a semiconductor.